The present invention relates to a drive element such as a driving wheel for goods-handling carts, in particular pallet trucks or the like, or such as a hoisting winch.
Of the various ways in which goods-handling carts can be motor-driven, the most widespread makes use of a driving wheel.
In most cases, the driving wheel can also be steered, being mounted to swivel about a vertical axis.
The swivel diameter of the wheel must be as small as possible, in particular when the driving wheel is not central, so as to maximize the distance between the wheels of the cart and increase its stability.
Cart manufactures often require the swivel diameter to be the same as the diameter of the wheel""s tire.
In known driving wheels, the wheel is rotated via a stepdown gear box by an electric motor of the direct current (DC) type or of the asynchronous type, which means that the volume occupied by the motor must be located above the wheel if the requirements of cart builders are to be satisfied.
This gives rise to a large amount of space being occupied in the vertical direction and to a structure that is relatively complex.
A similar space-occupancy problem arises for hoisting winches where the motor drives a drum.
European patent application No. EP-A-1 001 507 discloses an electrical machine for installing in a wheel of a private vehicle, the machine having a flux-concentrating rotor with permanent magnets.
An object of the present invention is to propose a novel drive element such as a driving wheel or a hoisting winch having a configuration that is compact and a structure that is relatively simple.
In this novel drive element the electric motor for driving the wheel or the drum is a synchronous motor having permanent magnets and a stator wound on teeth.
The term xe2x80x9cstator wound on teethxe2x80x9d is used to mean a stator in which each tooth carries its own individual winding, unlike stators where windings are interleaved and which present the drawback of greater axial size because of the relatively great length of the winding ends, due to the way in which the winding is performed. Furthermore, the number of teeth nteeth is preferably a function of the number of pairs of poles npairs and of the number of phases nphases in application of the formula nteeth=npairsxc3x97nphases.
The rotor is preferably a flux-concentrating rotor, i.e. the magnets are placed between pole pieces. Combining a flux-concentrating rotor with a stator wound on teeth makes it possible to have a machine that is relatively powerful while also being compact, thus making it possible in particular to mount the motor so that it is cantilevered-out on the end of a shaft, thereby reducing the number of ball bearings. The rotor can rotate at high speed since the magnets are protected from the pulsating flux by the pole pieces. There is therefore no need to use fragmented magnets at high speeds, as is the case with rotors whose magnets are mounted on the surface.
Advantageously, the magnets of the rotor are wedge-shaped when the machine is observed along the axis of rotation of the rotor, of width that tapers going away from the axis of rotation of the rotor, and the pole pieces present cutouts and are engaged by means of those cutouts on ribs of the shaft. They are thus fixed to the rotor shaft by means of complementary shapes. Co-operation between the wedge-shaped magnets and the pole pieces connected to the shaft by means of complementary shapes is particularly advantageous in that, at high speeds, the magnets can exert compression forces on the pole pieces tending to oppose splaying out of those regions of the pole pieces that are situated on either side of the ribs on which they are engaged. This makes it possible to avoid the need to engage the pole pieces on bars, unlike the structure described in U.S. Pat. No. 5,091,668.
In addition, the fact of placing magnets between pole pieces enables the magnet to be protected from the currents that are induced at high speeds of rotation, and thus to avoid the magnets becoming heated which could demagnetize them.
In the invention, the motor whose axial size is small can be placed inside a wheel, a drum, or a pulley, with the axis of the rotor preferably coinciding with the axis of rotation of the wheel, drum, or pulley. For a driving wheel, the motor no longer needs to be offset upwards and the vertical size of the driving wheel is small, thereby making additional space available to cart builders above the wheel.
The invention also makes it possible to simplify the structure of any gearing that might be used, which gearing can comprise a single stepdown stage, and be constituted by an epicyclic gear train, for example.
The stepdown ratio can be less 15, e.g. lying in the range 7 to 12, by using a permanent magnet rotor and a stator wound on teeth, making it possible to work both in attraction and in repulsion, and thus to generate greater torque than can be generated by a reluctance motor which works in attraction only.
The teeth of the stator can advantageously receive individual coils presenting heads that are very short, thus enabling axial size to be small. The teeth of the stator then preferably lack any pole spreading, thus making it easier to install the coils on the teeth, and also to remove them should that be necessary in order to replace them.
The use of a stator wound on teeth is also advantageous in that it makes it possible for the motor to be implemented with a relatively small number of teeth and coils, and thus at a cost which is advantageous. The mass of the magnets used can also be relatively low, thus reducing materials costs.
In a particular embodiment, the drive element comprises a main casing having a tubular wall of large diameter, a tubular wall of small diameter on the same axis as the preceding wall, and a transverse wall interconnecting said tubular walls. The stator is housed in the large-diameter tubular wall which is secured to a first bearing-forming piece. The small-diameter tubular wall is secured to a second bearing-forming piece. The large and small-diameter tubular walls and the transverse wall are preferably made as a single piece. In a variant, the large-diameter tubular wall can be constituted by a fitted piece, with the small-diameter tubular wall and the transverse wall being made as a single piece.
In a particular embodiment, the second bearing-forming piece also constitutes a planet-carrier having at least one shaft on which there rotates at least one planet wheel, and preferably three planet wheels, each planet wheel meshing both with a gear made in or fitted to the shaft of the rotor and with a ring gear of a support. This ring can be fitted to the support or it can be made integrally therewith. For a driving wheel, a rim supporting the tire of the wheel can be fixed to said support.
In a particular embodiment, the drive element includes a bearing enabling the support to turn on the small-diameter tubular wall of the main casing.
Still in a particular embodiment, the drive element includes a cap fitted in sealed manner onto the support. The cap may serve to contain grease or oil for lubricating the gearing.
For a driving wheel or a winch, the drive element preferably includes an electromagnetic brake that operates on power failure and that is fitted to the first bearing-forming piece, said brake comprising a disk fixed on a fluted end of the rotor shaft. The main casing can be swivel-mounted, or in a variant it can be fixed directly to the chassis when the drive element is a driving wheel that does not need to be steerable.
As mentioned above, the invention enables the rotor to be coaxial with the tire of the wheel or with the drum, thereby reducing the space occupied in a vertical direction by the drive element since there is no need for the motor to be offset.
The invention also provides a cart fitted with at least one driving wheel as defined above, which wheel can optionally be steerable.